We could make apathy a core value around here, if only we gave a flying [fig].
(OK, this was a collaborative effort, but I am claiming credit.)
Pretentious yes, pithy no. Drop either sentence to make it “quotable,” though I disagree both ways.
Not original with me, but I like “Time wounds all heels”.
“No good deed goes unpunished.” I don’t know who first said it. I first heard it in the song “Crumbling Down” by John Mellencamp.
“Nothing bad ever happens. Until it does.”
I like to say that to people who whine about doing stupid or thoughtless things and then whine about how it was never a problem before. Because they’ve done it any number of times before. And nothing bad ever happened before.
Mine’s a variation, “I trust him as far as I can throw myself.”
I say “we’ll burn that bridge when we come to it,” although I’m sure a lot of other people have come up with the same thing.
That’s A.E. Housman, from the poem Terence, This Is Stupid Stuff
How about “there was no greater loss, than when Heaven ceased to exist.”
“Winners never quit, and don’t trust whitey.”
[Thank you Lois Griffin]
The way you dance is the way you fuck, the way you feel is the way you walk.
Said to my stepmother in the midst of an argument:
“You wanna know a difference between you and me? People will cry at my funeral.”
Not exactly a Power Of Positive Thinking moment, but it remains true to this day.
“At times it is tempting to ignore those data that contradict our preconceived ideas.” I got this from a book, but if I could eliminate the cases of this I deal with, I think I would be a lot more satisfied.
This is something I wish my dad would think about. He has almost no real friends, and makes me almost routinely think about just driving away when I stay over. He’s never wrong, and his list of possible offenses is something I’ve never run into in other situations.
I’m not worried about what you intend to do; I’m worried about what you’re actually going to do.
I usually say, “We’ll jump off that bridge when we come to it.” Just seems more energetic, you know? 
My quote is quite useful if you have just hit your thumb with a hammer. It consists of just one word in different forms. Spoiler for those offended by profanity.
Fuck! fucked fucking fuckers, fuckingly fuck fucked fucks.
I think it is a grammatically correct sentence. My spell checker flags one word though. 
“All problems can be solved by the suitable application of high explosives, however, some are more fun when solved with a chainsaw.”
That’s original with me, although it started as a paraphrase from a book by Arthur C. Clarke, and evolved to the final form over time.
“It’s not who you know, it’s who you know knows.”
Could it possibly be the liquor? 